1. Field of the Invention
This invention relates to methods for producing gallium nitride (Al, Ga, In)N single crystal substrates that are useful for producing optoelectronic devices (such as light emitting diodes (LEDs), laser diodes (LDs) and photodetectors) and electronic devices (such as high electron mobility transistors (HEMTs)) composed of III-V nitride compounds.
2. Description of the Related Art
Group III-V nitride compounds such as aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), and alloys such as AlGaN, InGaN, and AlGaInN, are direct bandgap semiconductors with bandgap energy ranging from about 0.6 eV for InN to about 6.2 eV for AlN. These materials may be employed to produce light emitting devices such as LEDs and LDs in short wavelength in the green, blue and ultraviolet (UV) spectra. Blue and violet laser diodes may be used for reading data from and writing data to high-density optical discs, such as those used by Blu-Ray and HD-DVD systems. By using proper color conversion with phosphors, blue and UV light emitting diodes may be made to emit white light, which may be used for energy efficient solid-state light sources. Alloys with higher bandgaps may be used for UV photodetectors that are insensitive to solar radiation. The material properties of the III-V nitride compounds are also suitable for fabrication of electronic devices that may be operated at higher temperature, or higher power, and higher frequency than conventional devices based on silicon (Si) or gallium arsenide (GaAs).
Most of the III-V nitride devices are grown on foreign substrates such as sapphire (Al2O3) and silicon carbide (SiC) because of the lack of available low-cost, high-quality, large-area native substrates such as GaN substrates. Blue LEDs are mostly grown on insulating sapphire substrates or conductive silicon carbide substrates. Sapphire belongs to the trigonal symmetry group, while SiC belongs to the hexagonal symmetry group. GaN films and InGaN films have been heteroepitaxially grown on the c-plane sapphire surface for LED devices. Due to lattice mismatch, the GaN films grown on both sapphire and SiC substrates typically have high crystal defects with a dislocation density of 109 to 1010 cm−3. Despite the high defect density of the LEDs grown on these substrates, commercial blue LEDs produced from these materials have long lifetimes suitable for some applications.
UV LEDs based on alloys of GaN, however, show strong dependence of the power output on the substrate material used. The UV LEDs can be grown on native GaN substrate or on foreign substrates such as sapphire and silicon carbide. On the foreign substrate, a GaN or AlGaN thin film is first grown by utilizing appropriate techniques and the active UV LED structure is subsequently grown. It has been found that the power output of UV LEDs grown on native GaN substrate is much greater than the power output of those grown foreign substrates (see, for example, Yasan et al. Applied Physics Letters, Volume 81, pages 2151-2154 (2002); Akita et al. Japanese Journal of Applied Physics, Volume 43, pages 8030-8031 (2004)). The lower density of crystal defect of the device structure grown on native GaN substrate contributes to higher power output.
Group III-V nitride-based laser diodes also show a remarkable dependence of lifetime on the crystal defect density. The lifetime of these LDs dramatically decreases with the increase of the dislocation density (see, for example, “Structural defects related issues of GaN-based laser diodes,” S. Tomiya et al., MRS Symposium Proceedings, Vol. 831, p. 3-13, 2005). Low-defect density single-crystal gallium nitride substrates are needed for the long lifetime (>10,000 hours) nitride laser diodes.
Because of the very high equilibrium nitrogen pressure at the melting point, gallium nitride single crystals cannot be grown with conventional crystal growth methods such as the Bridgman method or Czochralski method where single crystals are grown from the stoichiometric melt. At ambient pressure, GaN starts to decompose well before melting.
Hydride vapor phase epitaxy (HVPE) has been utilized to grow relatively thick GaN on foreign substrates. In the HVPE process, gallium chloride (GaCl), formed by reacting gaseous hydrochloric acid (HCl) with gallium metal upstream in the reactor, is transported to the crystal growth region where it reacts with ammonia, depositing GaN on the surface of a substrate. The size of the GaN crystal grown may be the same size as the substrate. Substrates such as sapphire, gallium arsenide, silicon carbide, and other suitable foreign substrates have been used.
Vaudo et al. in U.S. Pat. No. 6,440,823 discloses a method of producing low defect GaN using HVPE on sapphire substrates. The sapphire substrate can be removed to produce a large area GaN substrate, for example, by a laser induced liftoff process as described by Kelly et al. (“Large freestanding GaN substrates by hydride vapor phase epitaxy and laser-induced liftoff,” Jpn J. Appl. Phys., Vol. 38, L217-L219, 1999). The wavelength of the laser beam, or the energy of the laser beam, is chosen so that it is smaller than the bandgap of the substrate, but larger than the bandgap of GaN. The substrate is transparent to the laser beam, but the GaN absorbs the laser energy, heating the interface and decomposing the GaN at the interface, which separates the GaN film from the substrate. In U.S. Pat. App. Pub. No. 2002/0068201, Vaudo et al. further discloses a method of producing freestanding GaN near the growth temperature by shining a laser beam at the interface between the grown GaN layer and the template, and decomposing the interface material. This process involves dangerous high-energy laser beams and high manufacturing cost.
Chin Kyo Kim in U.S. Pat. No. 6,923,859 discloses an apparatus and associated manufacturing method for GaN substrates in which a substrate and a GaN layer are separated from each other after growing the GaN layer on the substrate in the same chamber. The apparatus contains a transparent window at the circumference of the chamber to allow the introduction of the laser beam to the substrate. This process likewise involves dangerous high-energy laser beams and has high manufacturing cost.
Bong-Cheol Kim in U.S. Pat. No. 6,750,121 discloses an apparatus and method for forming a single crystalline nitride substrate using hydride vapor phase epitaxy and a laser beam. After growth of the GaN film on sapphire substrate, the wafer is moved to a heated chamber for laser-introduced separation. Because the wafer does not cool to room temperature, cracking induced by the mismatch of the coefficient of thermal expansion is eliminated. This process likewise involves dangerous high-energy laser beams and has high manufacturing cost.
Park et al. in U.S. Pat. No. 6,652,648 discloses a method of producing GaN substrate by first growing HVPE GaN on sapphire substrates. The backside of the sapphire substrate is protected for minimal parasitic deposition. After GaN growth on sapphire substrate, the GaN/sapphire structure is removed from the reactor. The GaN layer is subsequently separated from the sapphire substrate by a laser liftoff process. In addition to involving dangerous high-energy laser beams, the GaN layer on sapphire is likely to crack upon cool down, and thus this process suffers with low yield and high manufacturing cost.
Motoki et al. in U.S. Pat. No. 6,693,021 discloses a method of growing thick GaN film on gallium arsenide (GaAs) substrate. The GaAs substrate is wet-etched away to produce a free-standing GaN substrate. However, GaAs substrates tend to thermally decompose at the GaN growth temperature and in the GaN crystal growth environment, introducing impurities to the GaN film.
Yuri et al. in U.S. Pat. No. 6,274,518 discloses a method for producing a GaN substrate. A first semiconductor film (AlGaN) layer is formed on a sapphire substrate, and a plurality of grooves is formed on the AlGaN layer. A relatively thick GaN film is grown on a grooved AlGaN template by an HVPE method, and upon cooling down from the growth temperature to room temperature, GaN separates from the template, forming a large area freestanding GaN substrate. However, this method requires deposition and patterning of several films in different systems and cracking-separation. Thus, the process is one of low yield and high manufacturing cost.
Solomon in U.S. Pat. No. 6,146,457 discloses a thermal mismatch compensation method to produce a GaN substrate. The GaN film is deposited at a growth temperature on a thermally mismatched foreign substrate to a thickness on the order of the substrate, where the substrate is either coated with a thin interlayer or patterned. After cool down from the growth temperature to the room temperature, it is claimed that thermal mismatch generates defects in the substrate, not in the GaN film, producing a thick high quality GaN material. However, the GaN material of Solomon's invention is still attached to the underlying substrate, with the underlying substrate containing substantial defects and/or cracks. Subsequently, other processing steps are required to create a freestanding GaN layer.
Usui et al. in U.S. Pat. No. 6,924,159 discloses a void assisted method to manufacture GaN substrate. In this method, a first GaN thin film is deposited on a foreign substrate, and a thin metal film such as titanium film is then deposited on the first GaN thin film. The titanium metal film is heated in hydrogen-containing gas to form voids in the first GaN thin film. A thick GaN film is subsequently deposited on the first void-containing GaN film. The voids in the first GaN film create fracture weakness, and upon cooling from the growth temperature to ambient room temperature, the thick GaN layer separates itself from the substrate, forming a GaN wafer. However, this method requires deposition of several layers of films in different systems and cracking-separation. Thus, the process is one of low yield and high manufacturing cost.
The techniques of the prior art for manufacturing GaN wafers are attended by high manufacturing cost. There are some commercial vendors currently selling 2″ GaN wafers, but at very high price, reflecting the high manufacturing cost. Additionally, researchers have shown that the freestanding GaN substrates formed using the laser-induced liftoff process can be subject to substantial bowing, which limits their usability for device manufacturing (see, for example, “Growth of thick GaN layers with hydride vapour phase epitaxy,” B. Monemar et al., J. Crystal Growth, 281 (2005) 17-31).
In view of such prior-art approaches to forming GaN substrates, it is well-acknowledged that there is still a need in the art for low-cost methods for producing high-quality GaN substrates.